It has long been desired to provide improved microphone performance in devices such as communication devices and voice recognition devices that operate under a variety of different ambient noise conditions. Communication devices supporting hands-free operation permit the user to communicate through a microphone of a device that is not held by the user. Because of the distance between the user and the microphone, these microphones often detect undesirable noise in addition to the user's speech. The noise is difficult to attenuate and can be troublesome in vehicle applications due to the dynamically varying ambient noise present in the “cab” of the vehicle. For example, bi-directional communication systems such as two-way radios, cellular telephones, satellite telephones, and the like, are used in vehicles, such as automobiles, trains, airplanes and boats. It is preferable for the communication devices of these systems to operate hands-free, such that the user need not hold the device while talking, even in the presence of high ambient noise levels subject to wide dynamic fluctuations.
Bi-directional communication systems typically include both an audio speaker and a microphone. In order to improve hands-free performance in a vehicle communication system, a microphone is typically mounted near the driver's head. For example, a microphone is commonly attached to the vehicle visor or headliner using a fastener such as a clip, adhesive, hook-and-loop fastening tape (such as VELCRO brand fastener) or the like. The audio speaker associated with the communication system is preferably positioned remote from the microphone to assist in minimizing feedback from the audio speaker to the microphone. It is common, for example, for the audio speaker to be located in a vehicle adaptor, such as a hang-up cup or a cigarette lighter plug used to provide energizing power from the vehicle electrical system to the communication device or one or more of the speakers used by the radio. The position of the microphone as well as the microphone arrangement relative to the person speaking will determine the level of the speech signal output by the microphone and may affect the signal-to-noise ratio.
One potential solution to avoid these difficulties is disclosed in U.S. Pat. No. 4,930,742, entitled “REARVIEW MIRROR AND ACCESSORY MOUNT FOR VEHICLES,” issued to Schofield et al. on Jun. 5, 1990, which uses a microphone in a mirror mounting support. Although locating the microphone in the mirror support provides the system designer with a microphone location that is known in advance, and avoids the problems associated with mounting the microphone after the vehicle is manufactured, there are a number of disadvantages to such an arrangement. Because the mirror is positioned between the microphone and the person speaking into the microphone, a direct unobstructed path from the user to the microphone is precluded.
U.S. Pat. Nos. 5,940,503, 6,026,162, 5,566,224, 5,878,353, and D402,905 disclose rearview mirror assemblies with a microphone mounted in the bezel of the mirror. None of these patents, however, discloses the use of acoustic ports facing multiple directions nor do they disclose microphone assemblies utilizing more than one microphone transducer. The disclosed microphone assemblies do not incorporate sufficient noise suppression components to provide output signals with relatively high signal-to-noise ratios. Moreover, they do not provide microphones having a directional sensitivity pattern nor do they have a main lobe directed forward of the housing for attenuating signals originating from the sides of the housing or undesired locations.
It is also highly desirable to provide voice recognition systems in association with vehicle communication systems, and most preferably, such a system would enable hands-free operation. Hands-free operation of a device used in a voice recognition system is a particularly challenging application for microphones since the accuracy of a voice recognition system is dependent upon the quality of the electrical signal representing the user's speech. Conventional hands-free microphones are not able to provide the consistency and predictability of microphone performance needed for such an application in a controlled environment such as an office as well as an uncontrolled and/or noisy environment such as an automobile.
Commonly-assigned U.S. Patent Application Publication Nos. 2004/0208334-A1 and 2002/0110256-A1 and PCT Application Publication No. WO 01/37519 A2, which are herein incorporated by reference, disclose various embodiments of rearview mirror-mounted microphone assemblies. In those embodiments, at least one microphone transducer is typically aimed at the driver of the vehicle. This usually results in the microphone assembly receiving audible voice and noise from all directions within the vehicle cab. Since noise may be introduced into the microphone from anywhere within the vehicle, this raises many types of performance issues when used in certain environments and in combination with digital signal processing circuits. Those skilled in the art will also recognize that there are a number of microphone array placement techniques that are known to offer improved signal-to-noise performance. These techniques typically combine the output of two or more unidirectional microphones to achieve a superior signal in noise conditions.
Prior art FIG. 1 illustrates a side fire four microphone array where a two element side fire array is optimally arranged so as to achieve directional gain from the side of the array. Similarly, FIG. 2 illustrates an end fire four microphone array where the omni-directional microphones are oriented to achieve their best performance from audio coming from the array's end. Although these arrangements work to achieve gain in a predetermined direction, they also work to attenuate noise coming from directions other than those which they are optimized. Using these omni-directional microphone arrangements can achieve results substantially equivalent to that of a first order directional microphone. Thus, it would be necessary to use the equivalent of four omni-directional microphones to achieve the same results as the two directional microphones in these array configurations.
Yet in other applications, it is known to replace two directional units with four omni-directional microphones. However, when processed omni-directional microphones are used to replace directional microphones, there is also an additional advantage of optimized polar patterns and an ability to create first and second order directionality using various frequency combinations. Moreover, greater audio processing is often required since these types of microphone arrangements can have low frequency signal-to-noise problems.
Accordingly, a microphone assembly is contemplated for a vehicle that will provide improved hands-free performance for enabling voice recognition operation when a digital signal processing circuit is utilized. Additionally, the microphone assembly should be directive for use in a specific spatial location within a vehicle while using only a limited number of omni-directional microphone transducers.